TC bananas bear fruit

Demand for tissue culture bananas has seen nurseries springing up across sub-Saharan Africa.Africa Harvest

An icon of the East African landscape is the highland banana, growing around most homesteads. An important staple, the crop is relatively hardy and easy to grow but is plagued by numerous pests and diseases. Consequently, highland bananas rarely yield to their potential. Micropropagation techniques, such as production of tissue culture (TC), have been hailed as one means of providing commercial and smallholder growers with higher yielding, better quality planting material. But TC is not yet the panacea to the low productivity of African bananas.

Increasing demand for TC banana, and other crops, has seen a number of public and private laboratories as well as plant nurseries springing up across sub-Saharan Africa to provide improved planting material for a variety of farming systems. Uganda, Kenya, Burundi, Nigeria, Mali, Cote d'Ivoire, Ghana, Cameroon and South Africa, all report having TC facilities.

Better staff, better standards

In Burundi, private company Agrobiotec multiplies around 250,000 plantlets of more than 40 cultivars for distribution to farmers across the country. Despite plans to construct a new laboratory in 2009, director Theodomir Rishirumuhirwa states that the company faces a variety of constraints, including inadequate laboratory infrastructure, unskilled personnel and a lack of chemicals. Agro-Genetic Technologies Ltd, the first and currently only private TC company in Uganda faces similar issues. "We have a particular problem in finding the necessary skilled personnel for our work," says managing director Erostus Nsubuga. "Maintaining laboratory facilities and sourcing adequate materials are also challenges."

Prof. Esther Murugi Kahangi, a TC expert and head of research and development at Jomo Kenyatta University of Agriculture and Technology (JKUAT) in Kenya, believes that after eight years of TC banana availability in the country, "there is now an urgent need to develop reliable standards and to train quality laboratory staff and inspectors."

Despite the increase in TC facilities across Africa, Kahangi points out that very few private enterprises exist and that the majority of micropropagated bananas continue to be produced by public research institutions. She is particularly concerned at the lack of virus indexing facilities - the ability to test for viruses in planting material. Whilst TC banana plantlets are produced in a sterile environment, and are free of fungal and bacterial diseases, viruses can still be transmitted in the cloned planting material.

Micropropagation can provide large numbers of high-quality, disease-free plantlets.Africa Harvest

East Africa is currently free of several key viral diseases, including banana bract mosaic virus (BBrMV) and banana virus X (BVX), but banana bunchy top virus (BBTV) has recently been identified in Rwanda (see Rearing its ugly head - bunchy top disease), posing a significant threat to neighbouring countries. Of particular concern is the spread of infected planting material between farmers, who have a tradition of exchanging banana suckers for propagation. But, with the rise in multiplication of TC bananas, this too has the potential to increase the distribution of potentially virus-infected plantlets. To address this issue, scientists from Uganda, Kenya, and Tanzania are currently working with the Africa Harvest Biotech Foundation under the Gates Foundation-funded Grand Challenges in Global Health programme, to establish key virus indexing programmes within key TC laboratories.

Soil fertility, a major challenge...

But, even with clean planting material, TC bananas will not achieve optimum yields, states Kahangi, if soil fertility is poor. "Soil fertility is a major problem for bananas," she says. "The lack of soil nutrients for bananas is far more important than losses from pests and disease, or the impact of farmer management practices."

Tissue culture and micropropagation can improve access to high yielding banana varieties.Africa Harvest

Kahangi's current work is focusing on re-introducing some of the beneficial micro-organisms that are eliminated during the sterile process of TC propagation; these include mycorrhizae fungi, which can help make better use of available soil nutrients. Introduction of fungal endophytes could also help to control pests such as nematodes and weevils.

Sustainable propagation media is also an issue for the expansion of TC multiplication. In Kenya, forest soil has generally been used for potting seedlings, but it is a limited resource and is expensive to sterilise. Furthermore, the Kenya forestry department has recently declared restrictions on soil extraction. Coco peat, used in flower production, is one alternative: it can be used for up to four years and is slow to disintegrate. Coconut coir is another option, but more research is needed to determine the potential for these media as some can stifle growth if plantlets are left in pots for too long.

...and credit, too

Whilst research into 'next generation' TC technologies is a priority, Kahangi states that credit will continue to constrain farmers from purchasing improved planting material and the inputs required to achieve optimum yields. Agronomic practices required for maximising the benefits of TC bananas also need to be better documented and disseminated. And while banana markets remain underdeveloped, postharvest losses are high and discourage farmers from adopting the technology.